A spool valve in which flow paths between a plurality of ports are opened and closed by a spool sliding in a valve hole is well known as disclosed in, for example, Patent Literature 1. As shown in FIGS. 10 and 11, a conventional general spool valve 60 has, in a housing 61 having a plurality of ports P, A1, A2, E1, and E2, a valve hole 62 having a circular cross-section and communicating with the ports P, A1, A2, E1, and E2, and is configured such that a spool 63 is accommodated in the valve hole 62 so as to be slidable in the direction of the axis L, a pilot valve portion 67 is provided at the end portion of the housing 61, the pilot valve portion 67 is provided with a pilot piston 69 and a pilot valve 68 that causes a pilot fluid to act on the pilot piston 69, and the spool 63 is switched by the pilot valve 68 via the pilot piston 69.
The spool 63 has land portions 64 that have a circular cross-section and to the outer peripheries of which seal members 65 are attached, and shaft portions 66 that have a circular cross-section and a smaller diameter than the land portions 64 and that connects the adjacent land portions 64, and slides in the valve hole 62 so that the land portions 64 ride on valve seat portions 62a formed in parts of the valve hole 62 and separate from the valve seat portions 62a, and thereby opens and closes flow paths connecting the adjacent ports P, A1, A2, E1, and E2.
In such a spool valve, in order to increase the flow rate of the controllable pressure fluid, it is necessary to increase the effective cross-sectional area of the flow paths. Examples of methods therefor include increasing the diameter of the valve hole 62 and the spool 63 (land portions 64), and increasing the stroke of the spool 63 and the opening width of the ports P, A1, A2, E1, and E2.
Among them, the former method in which the diameter of the valve hole 62 and the spool 63 is increased can easily and reliably increase the effective cross-sectional area of the flow paths, but along with this, the lateral width of the housing 61, that is, the valve width is increased, which leads to an increase in the size of the spool valve. This method is not preferable under circumstances requiring a spool valve whose size is as small as possible and whose flow rate as high as possible.
On the other hand, the method in which the stroke of the spool 63 and the opening width of the ports P, A1, A2, E1, and E2 are increased has an advantage that it is not necessary to increase the lateral width of the housing 61, which hardly leads to an increase in size of the spool valve. However, since the diameter of the valve hole 62 and the spool 63 are unchanged, the cross-sectional area of the flow paths around the shaft portions 66 of the spool 63, that is, the cross-sectional area S0 of the flow paths between the inner periphery of the valve hole 62 and the outer peripheries of the shaft portions 66 is not increased. Therefore, it is inevitable to be largely restricted by the cross-sectional area of the flow path around the shaft portions, and it has been difficult to greatly increase the effective cross-sectional area of the entire flow path.